This application is based on, and claims priority of, Canadian Patent Applications Nos. 2,326,362 filed Nov. 20, 2000, and 2,327,862 filed Dec. 6, 2000.
Not Applicable.
The present application relates to the field of optical cross-connects, and in particular to a cascadable optical cross-connect.
Optical matrix cross-connects (or switches) are commonly used in communications systems for transmitting voice, video and data signals. Generally, optical matrix cross-connects include multiple input and/or output ports and have the ability to connect, for purposes of signal transfer, any input port/output port combination, and preferably, for Nxc3x97M switching applications, to allow for multiple connections at one time. At each port, optical signals are transmitted and/or received via an end of an optical waveguide. The waveguide ends of the input and output ports are optically connected across a switch core. In this regard, for example, the input and output waveguide ends can be physically located on opposite sides of a switch core for direct or folded optical pathway communication therebetween, in side-by-side matrices on the same physical side of a switch core facing a mirror, or they may be interspersed in a single matrix arrangement facing a mirror.
Establishing a connection between any input port and a selected output port involves configuring an optical pathway across the switch core. One known way to configure the optical pathway is by moving or bending optical fibers using, for example, piezoelectric actuators. The actuators operate to displace the fiber ends so that signals from the fibers are targeted at one another so as to form the desired optical connection across the switch core. The amount of movement is controlled based on the electrical signal applied to the benders. By appropriate arrangement of actuators, two-dimensional targeting control can be effected.
Another way of configuring the optical path between an input port and an output port involves the use of one or more movable mirrors interposed between the input and output ports. In this case, the waveguide ends remain stationary and the mirrors are used to deflect a light beam propagating through the switch core from the input port to effect the desired switching. Micro-electro-mechanical mirrors known in the art can allow for two-dimensional targeting to optically connect any input port to any output port. For example, U.S. Pat. No. 5,914,801, entitled MICROELECTROMECHANICAL DEVICES INCLUDING ROTATING PLATES AND RELATED METHODS, which issued to Dhuler et al on Jun. 22, 1999; U.S. Pat. No. 6,087,747, entitled MICROELECTROMECHANICAL BEAM FOR ALLOWING A PLATE TO ROTATE IN RELATION TO A FRAME IN A MICROELECTROMECHANICAL DEVICE, which issued to Dhuler et al on Jul. 11, 2000; and U.S. Pat. No. 6,134,042, entitled REFLECTIVE MEMS ACTUATOR WITH A LASER, which issued to Dhuler et al on Oct. 17, 2000, disclose micro-electro-mechanical mirrors that can be controllably moved in two dimensions to effect optical switching. U.S. Pat. No. 6,097,858, entitled SENSING CONFIGURATION FOR FIBER OPTIC SWITCH CONTROL SYSTEM, and U.S. Pat. No. 6,097,860, entitled COMPACT OPTICAL MATRIX SWITCH WITH FIXED LOCATION FIBERS, both of which issued to Laor on Aug. 1, 2000, disclose switch control systems for controlling the position of two-dimensionally movable mirrors in an optical switch.
An important consideration in cross-connect design is minimizing physical size for a given number of input and output ports that are serviced, i.e., increasing the packing density of ports and beam directing units. It has been recognized that greater packing density can be achieved, particularly in the case of a movable mirror-based beam directing unit, by folding the optical path between the ports and the movable mirror and/or between the movable mirror and the switch interface. Such a compact optical matrix switch is disclosed in U.S. Pat. No. 6,097,860. In addition, further compactness advantages are achieved therein by positioning control signal sources outside of the fiber array and, preferably, at positions within the folded optical path selected to reduce the required size of the optics path.
Another technique of minimizing cross-connect size is to minimize the number of mirrors within the switch core. For example, a typical two-dimensional (2D) switch core employs a matrix of N2 independently movable mirrors, which enable Nxc3x97N switching using a single reflection of a light beam propagating between an input port and a selected output port. However, since each of the N2 mirrors must be independently controlled, increases in the size of the switch core (that is, increasing N) implies an exponential increase in the number of mirrors, and a corresponding increase in the size of the switch control circuitry. These factors combine to impose practical limits on the size of the cross-connect. One way of overcoming this problem, and thereby enabling the construction of high capacity cross-connects, is to employ a three-dimensional (3D) switch core, in which 2N independently movable mirrors are used to enable Nxc3x97N switching. Within a 3D switch core, optical switching is performed using two reflections of the light beam propagating between an input port and a selected output port. The reduced number of mirrors required by a 3D switching core (2N as compared to N2 required by a 2D switching core) is beneficial in that it permits the construction of a physically smaller switch core. Additionally, it permits a corresponding reduction in the size of the switch control system, and therefore enables construction of higher capacity cross-connects.
While it is desirable to minimize the physical size of the cross-connect, a competing demand is to maximize the number of fibers (or waveguides) between which signals can be switched. In electronic switching technologies, this is commonly accomplished by connecting a plurality of cross-connect blocks together to form a network having a larger capacity. As is known in the art, a fully non-blocking Mxc3x97M cross-connect can be created using a plurality of smaller (that is, lower capacity) Nxc3x97N cross-connect blocks interconnected to define a 3-layer Clos network.
Implementation of Clos network architectures in the optical domain requires that cross-connect blocks be cascaded. This, in turn, requires that each block be capable of Nxc3x972N switching. As is well known in the art, Nxc3x972N optical switching can be accomplished using a conventional 2D Nxc3x972N switch core, in which 2N mirrors are positioned within a propagation path of an input light beam to selectively switch the light beam to a respective one of 2N output ports. An analogous technique uses a switch core composed of a MEMS array having N independently positionable mirrors, each of which is designed to switch a respective incident light beam to a selected one of 2N output ports. Both of these approaches suffer from scalability limitations, as even small increases in the value of N result in sizable increases in either the number of mirrors (and thus the size of the switch core) or the number of positions to which each mirror must be positioned (and thus the complexity of the control system).
An alternative technique of implementing Nxc3x972N switching is to provide the switch core with express paths that couple inbound optical signals directly to an express output port (typically connected to an input port of an adjacent switch block) effectively bypassing the switch core. Such express paths can readily be implemented in a conventional 2D Nxc3x97N switch core, by positioning a respective express output port on the opposite side of the switch core from each input port. With this arrangement, when all of the N mirrors associated with any one input port are positioned out of the propagation path, the light beam from the input port will transit the switch core to the respective express output port unhindered. However, in a 3D switch core, no such direct paths through the switch core exist. Consequently, cross-connect blocks incorporating 3D switch cores cannot be cascaded, and thus are not utilized in large switch networks.
Accordingly, a high capacity optical cross-connect, which can be readily cascaded remains highly desirable.
Accordingly, an object of the present invention is to provide a high capacity cascadable optical cross-connect.
Thus an aspect of the present invention provides a cascadable optical cross-connect including a pair of optical arrays and a plurality of optical bypasses. Each optical array includes a respective plurality of mirrors. Each optical bypass is disposed within a respective optical array, and is adapted to permit a light beam to pass through the respective optical array between a respective waveguide and a respective mirror of the opposite optical array. The mirrors are moveable to selectively define a propagation path of a light beam through the optical cross-connect via any two optical bypasses.
Preferably, at least one optical bypass is disposed within one optical array, and at least two optical bypasses are disposed within the opposite optical array. Each optical bypass may be provided as any one of: an optically transparent region of the respective optical array; and an opening defining a passage through the respective optical array.
Each optical bypass may also include means for deflecting light beams propagating through the optical bypass. In such cases, the deflecting means may be provided as any one or more of: a lens; a prism; and a mirror. This arrangement enables the optical bypasses to be located close to one another, thereby facilitating a compact switch core. As light beams propagate through the optical bypass, they are deflected to an area of the cross-connect in which there is sufficient space to permit installation of waveguides to conduct the light beams away from the cross-connect.
Each optical bypass may be associated with a respective plurality of waveguides. In such cases, the optical bypass is preferably adapted to permit respective light beams to pass through the respective optical array between each one of the associated plurality of waveguides and the opposite optical array. Each optical bypass may include means for directing respective propagation paths of the light beams to converge as they pass through the optical bypass. The means for directing the propagation paths of the light beams may include a respective relay lens disposed between the optical bypass and the associated waveguides. In this case, the relay lens preferably has a focal point disposed within the optical bypass. Alternatively, the associated waveguides may be radially disposed about an optical axis of the optical bypass, such that light beams emerging from each waveguide converge as they pass through the optical bypass.
In some embodiments, a number of mirrors of one optical array is equal to a number of waveguides associated with each optical bypass of the opposite optical array. Each mirror of one optical array preferably lies in a propagation path from a respective one waveguide associated with each optical bypass of the opposite optical array. Thus each mirror can deflect a light beam to (or, equivalently, receive a light beam from) any one of a set of waveguides composed of one waveguide associated with each optical bypass of the opposite optical array.
In some embodiments, each one of the plurality of mirrors of each optical array includes a respective default position defining a propagation path between a selected pair of optical bypasses of the opposite optical array. Preferably, each mirror automatically moves to the respective default position in an event of a failure of the optical cross-connect. By this means, when a failure of the optical cross-connect occurs, light beams entering the optical cross-connect can be directed out of the cross-connect, for example to a protection cross-connect.
In some embodiments, each optical array further comprises an axis of symmetry. In such cases, at least the respective plurality of mirrors may be symmetrically disposed about the axis of symmetry. One of the optical bypasses of each optical array may be disposed on the respective axis of symmetry of the optical array. The axes of symmetry of each of the optical arrays are preferably co-extensive.
In some embodiments, a lens is disposed between the pair of opposed optical arrays. Preferably, the opposed optical arrays are oriented to lie in respective opposed focal planes of the lens. Still more preferably, at least one optical bypass of each optical array is disposed on an optical axis of the lens, and the respective plurality of mirrors of each optical array are symmetrically disposed about an optical axis of the lens.
Thus the present invention provides a high capacity cross-connect in which a 3D switch core includes multiple optical bypasses providing corresponding multiple propagation paths through the switch core to facilitate cascading of cross-connect blocks. A light beam propagating in a waveguide enters the switch core through an optical bypass through a first optical array and propagates to a mirror of a second optical array (opposite the first optical array). The mirror can deflect the light beam to either an express optical path or a switching optical path. In the express optical path, the light beam propagates from the mirror to a waveguide associated with another optical bypass of the first optical array. In the switching optical path, the light beam propagates from the mirror to a selected mirror of the first optical array, which deflects the light beam to a waveguide associated with a selected optical bypass of the second optical array.